Gas turbine power plant operators are often faced with an economic dilemma of whether or not to operate their plants during low power demand times. By operating continuously, the plant will be available to quickly produce base load power when the power demand becomes high. The plant maintenance cost will also be reduced with fewer plant starts and stops. However, operation during these low power demand times often results in negative profit margins, or losses, for the plant operator because the low cost of power does not offset the cost of fuel.
The logical solution for the plants that choose to operate continuously is to minimize the losses by minimizing fuel consumption during operation at minimum power demand. Industrial gas turbine engines are designed to operate at a constant design turbine inlet temperature under any ambient air temperature (i.e., the compressor inlet temperature). This design turbine inlet temperature allows the engine to produce maximum possible power, known as base load or a full load operating mode. Any reduction from the maximum possible base load power, such during a plant turn down, is referred to as a part load operating mode. That is, part load entails all engine operation from 0% to 99.9% of base load power. However, operation of the plant is restricted by its exhaust gas emissions permit. Since emissions such as nitrous oxides (NOx) and carbon monoxide (CO) typically increase on a volumetric basis as the gas turbine power decreases, this limits how much the plant can turn down, or reduce power, during the low power demand times.
In particular, part load operation may result in the production of high levels of carbon monoxide (CO) during combustion. One known method for reducing part load CO emissions is to bring the combustor exit temperature or the turbine inlet temperature near that of the base load design temperature. It should be noted that, for purposes of this disclosure, the terms combustor exit temperature and turbine inlet temperature are used interchangeably. In actuality, there can be from about 30 to about 80 degrees Fahrenheit difference between these two temperatures due to, among other things, cooling and leakage effects occurring at the transition/turbine junction. However, with respect to aspects of the present invention, this temperature difference is insubstantial.
To bring the combustor exit temperature closer to the base load design temperature, the mass flow of air through a turbine engine can be restricted by closing compressor inlet guide vanes (IGV), which act as a throttle at the inlet of a compressor for the gas turbine engine. When the IGVs are closed, the trailing edges of each of the vanes rotate closer to the surface of an adjacent vane, thereby effectively reducing the available throat area. Reducing the throat area reduces the flow of air which the first row of rotating blades can draw into the compressor. Lower flow to the compressor leads to a lower compressor pressure ratio being established in the turbine section of the engine. Consequently, the compressor exit temperature decreases because the compressor does not input as much energy into the incoming air. Also, the mass flow of air through the turbine decreases, and the combustor exit temperature increases.
While controlling emissions during plant turn down is effectively controlled by closing the IGVs, this has limited capability. Constant speed compressors, such as those used for industrial gas turbines, have limitations on the amount that the mass air flow into the compressor may be reduced using the IGVs before running into structural and/or aerodynamic issues. Further, CO emissions increase rapidly as gas turbine engine load is reduced below approximately 60%. Once the IGVs have been closed to their limit, and the engine's exhaust temperature limit has been reached, then power typically may be reduced only by decreasing turbine inlet temperature. Turbine inlet temperature reduction corresponds to a decrease in the combustion system's primary zone temperature, resulting in CO and unburned hydrocarbons (UHC) being produced due to quenching of the combustion reactions in the turbine hot gas path.